How do you square up a foundation?

Square up a foundation by measuring both diagonals of the rectangle and adjusting corners until the diagonals are equal. First, set batter boards at each corner and run string lines to form the rectangle. Use the 3-4-5 method (or a larger multiple like 12-16-20) to establish each 90-degree corner. Then measure both diagonals — from corner to opposite corner. If both diagonals match within 1/8", your foundation is square. If one is longer, shift that diagonal's corner inward until both measurements are equal.

How do you calculate the diagonal of a rectangle?

Calculate the diagonal of a rectangle using the Pythagorean theorem: diagonal = √(length² + width²). For example, a 10-foot by 12-foot rectangle has a diagonal of √(100 + 144) = √244 = 15.62 feet, or approximately 15' 7-1/2". Both diagonals of a rectangle are always equal in length. Use this calculator to get the exact measurement in feet-inches-fractions format for your tape measure.

What if my diagonals don't match?

If your diagonals don't match, the layout is racked — it's a parallelogram instead of a true rectangle. To fix it, identify which diagonal is longer. Push that diagonal's corner inward (toward the center of the rectangle) while keeping the opposite corner fixed. Re-measure both diagonals after each adjustment. For residential work, diagonals should match within 1/8". For precision work like cabinet installation or tile layout, match within 1/16".

What are common Pythagorean triples used in construction?

The most common Pythagorean triples in construction are 3-4-5, 5-12-13, and 8-15-17. The 3-4-5 triple is the most widely used because it's easy to remember and works at any scale. Common multiples include 6-8-10 (small decks), 9-12-15 (garages), 12-16-20 (house foundations), and 15-20-25 (commercial). The 5-12-13 and 8-15-17 triples are useful when your layout proportions are closer to those ratios.

How accurate does squaring need to be?

For most residential construction, squaring should be accurate to within 1/8" on the diagonal measurement. Foundation forms, deck framing, and wall layout all use this tolerance. For finish carpentry — such as cabinet installation, countertops, or tile layout — aim for 1/16" or better. The larger your 3-4-5 triangle, the more accurate the squaring will be. A 12-16-20 triangle provides four times the accuracy of a basic 3-4-5 for the same measurement error.

Can I use a laser measure to square a foundation?

Yes, a laser distance meter is excellent for squaring foundations, especially large ones. Laser measures are accurate to 1/16" at distances up to 300 feet, and they eliminate the sag and stretch errors that occur with long tape measures. Set the laser at one corner, aim at the opposite corner, and read the diagonal directly. Compare with the calculated diagonal from this calculator. For the 3-4-5 method, laser measures work well for the long sides but a tape measure is usually faster for the short 3-4 foot measurements.

Square-Up Calculator — Diagonal for Squaring

Q: What is the 3-4-5 rule in construction?

The 3-4-5 rule is a method for establishing a 90-degree angle using a tape measure. Measure 3 feet along one side of the corner and 4 feet along the other side. When the diagonal between those two points measures exactly 5 feet, the corner is a perfect right angle. This works because 3-4-5 is a Pythagorean triple: 3² + 4² = 9 + 16 = 25 = 5². For greater accuracy, use multiples like 6-8-10, 9-12-15, or 12-16-20.

How to Use the Square-Up Calculator

How to Square Up a Foundation, Deck, or Wall

Squaring a rectangular layout is one of the most fundamental tasks in construction. Whether you are pouring a foundation, framing a deck, building a fence, or laying out a wall, every rectangle must be square — meaning all four corners are exactly 90°. The fastest way to verify this is by measuring the diagonal. If both diagonals of a rectangle are equal, the rectangle is perfectly square. This calculator computes that diagonal for you using the Pythagorean theorem: diagonal = √(length² + width²). Enter your length and width and get the exact diagonal measurement in feet-inches-fractions, decimal, and metric formats.

The 3-4-5 Method for Squaring Corners

The 3-4-5 method is the most popular technique for establishing a square corner on a construction site. It works because 3, 4, and 5 form a Pythagorean triple — a set of three whole numbers where the sum of the squares of the two shorter sides equals the square of the longest side: 3² + 4² = 9 + 16 = 25 = 5². On site, you measure 3 feet along one side of the corner, 4 feet along the other, and adjust until the diagonal between those two points measures exactly 5 feet. When the diagonal hits 5', your corner is 90°.

For larger layouts, use a multiple of 3-4-5 for greater accuracy. The bigger the triangle, the smaller the effect of tape measure slop. Common multiples include:

6-8-10 — double the 3-4-5 (good for small decks and patios)
9-12-15 — triple (good for garage and room additions)
12-16-20 — quadruple (good for house foundations)
15-20-25 — quintuple (commercial and large foundations)

Other useful Pythagorean triples include 5-12-13 and 8-15-17, which are handy when your layout proportions don't fit the 3-4-5 ratio.

Step-by-Step: Squaring a Deck or Foundation

Set two batter boards at one corner of the layout. Run string lines to mark the two sides that meet at the corner. Where the strings cross is your corner point.
Measure the sides. Pull your tape along one string line and mark 3 feet (or 6, 9, 12, etc.). Then pull along the other line and mark 4 feet (or 8, 12, 16, etc.).
Measure the diagonal. Hook the tape on the 3' mark and stretch to the 4' mark. If the diagonal reads exactly 5 feet (or 10, 15, 20, etc.), the corner is square. If not, adjust the string line on the batter board until it does.
Verify with both diagonals. After squaring all four corners, measure both diagonals of the full rectangle. They should be equal. If one diagonal is longer than the other, the layout is racked (a parallelogram instead of a rectangle). Shift the long diagonal's corner inward until both diagonals match.

When to Use the Full Diagonal vs. the 3-4-5 Method

The 3-4-5 method is best for establishing individual corners when you don't yet have the full rectangle laid out. It is fast, requires only a tape measure, and works at any scale. Use it at each corner during initial layout.

The full diagonal measurement is the final verification step. After all four corners are set, measure both diagonals of the entire rectangle. If they match (within 1/8" for most residential work), the layout is square. This calculator gives you the exact diagonal so you know what number to expect on your tape before you pull it. That eliminates guesswork and saves time when the diagonal doesn't read a nice round number.

Pro Tips

Use the longest triangle you can when squaring with 3-4-5. A 12-16-20 triangle has four times the baseline of 3-4-5, so a 1/8" measurement error has one quarter the angular effect. For house foundations, 12-16-20 or 15-20-25 is standard.
Always check both diagonals. Matching diagonals is the only way to confirm that all four corners are square and the layout is a true rectangle, not a parallelogram.
On sloped ground, measure level distances, not surface distances. Hold one end of the tape at the high point and level it with a torpedo level or laser before reading the measurement. Sloped measurements will give a false diagonal.
For foundation layouts larger than your longest tape measure, use a laser distance meter. They measure up to 300+ feet with 1/16" accuracy and eliminate sag in long tapes.
Mark your diagonal on the tape with painter's tape or a Sharpie before going to the field. Knowing the exact target number ahead of time makes the adjustment process faster and more confident.

Free Square-Up Calculator — Diagonal for Squaring